Understanding the pathogenesis of neurological disease caused by arboviruses is fundamental to the treatment of lethal viral encephalitides using drug or vaccine therapy. Rift Valley Fever is a globally-important emerging zoonotic arboviral disease that presents a significant risk to humans and livestock communities. RVFV infection of humans can result in a spectrum of clinical outcomes, including acute febrile illness, severe hepatic disease, or delayed-onset encephalitis. The mechanisms behind the different human clinical outcomes are undefined and represent a void in our understanding of this emerging viral disease. The long-term goal is to understand the pathophysiologic mechanisms contributing to the spectrum of clinical outcomes after infection with RVFV as well as the immunological response essential for protection against RVF. The overall objectives of this proposal are to determine how neuroinvasion by Rift Valley Fever Virus occurs and the role of the host immune response in the neurological disease and outcome of infection. The central hypothesis for this proposal is that a neutrophil-mediated host inflammatory response in the brain exacerbates neuroinvasion and neurological disease in RVFV-infected rats. The central hypothesis will be addressed with the following Specific Aims: 1) Determine the route of neuroinvasion by RVFV. Since a large gap in knowledge exists as to how RVFV invades the central nervous system, the route of neuroinvasion from the site of infection to the brain will be determined. 2) Determine how neutrophil infiltration into the brain affects neurological disease. Preliminary data suggests that neutrophil recruitment contributes to immune-mediated tissue damage in the brain of infected rats. To test this hypothesis, neutrophil migration into the brain in response to RVFV infection will be measured, and neutrophils will be depleted or blocked from migrating to determine the effect on RVF disease course, pathogenesis and outcome. Upon completion of this study, a critical void will have been filled in the understanding of how RVFV enters the brain and the contribution of host neutrophils to pathogenesis of lethal encephalitic RVF.